In a large Dutch oven, melt butter over medium heat.,Add onion, and cook for 6-8 minutes or until tender.,______,Gradually add milk, and cook, stirring occasionally, until thickened and bubbly.,Add cubed cheese, Monterey Jack cheese, parsley, dry mustard, salt, and pepper; cook over low heat, stirring constantly until cheese is melted and mixture is smooth.,Add cooked pasta, stirring gently to combine.,Serve immediately; garnish with crumbled bacon or shredded cheddar cheese.